In today's spiritual/occult world a lot of emphasis is put on spiritual development, psychic abilities, magic (of the occult variety) and ghosts. It is this last category of phenomena that I would like to address in this article.
Just what is a ghost? That depends on how you qualify 'ghosts'. I put them in four categories:
1) disembodied spirits of human beings
2) disembodied spirits that were never human (ie: astral beings)
3) psychic impressions
4) astral travelers
The first category is what most people, including myself, think of as ghosts - the spirits of the deceased.
The second category is hard to define exactly, but can be thought of as entities whose whole existence - start to finish - is spent on the astral planes.
But I believe that what most people experience as ghosts are from the third category. These are impressions of highly emotional happenings that have been 'recorded' by the surroundings and are 'read' by those who are psychically sensitive enough to perceive them. This is why so many people report the same things happening again and again over long periods of time, like a movie being played over and over again. It also explains why such sightings usually fade out over a period of time. It would depend on the force of the emotions which impinged on the surroundings and what material(s) 'recorded' them, but most seem to die out after a period of 100-200 years. I cannot conceive of a "living" spirit being content to re-enact the same series of events that long. Boooooriiiiiing....
I'm quite sure in my own mind that no spirit of a deceased person would stay, at the longest, any longer than it would take for their dearest relatives to "pass over", but wouldn't be hanging around all the time, even then. Even so, I believe that most don't stick around any longer than it takes to take a look 'round at their relatives and other loved ones one last time and, possibly, try to reassure their loved one(s) that they're ok. ...Then, of course, there are the spirits of those who don't realize, or refuse to believe that, as physical entities, they're dead.
An astral traveller would be any living being that just happens to be travelling on the astral plane, whether while asleep or purposely. Categories two and four are seen occasionally, but we have a tendency to interpret anything out of the ordinary as a ghost, when category one is what we actually mean.
Earlier I mentioned highly emotional events being 'recorded' and the materials which record them. I'd like to clarify what I mean by this: When we think of ghosts, hauntings and such, we immediately think of violent events; murder, executions, suicide, etc. While these definitely figure in, we must also include long periods of great joy (such as is found with children), deep, lingering sorrow, the emotions and pain engendered from long, drawn-out and/or painful deaths from long illnesses, cancer, etc., or even non-fatal occurences of same, such as is seen in some of the tales where a person suffers in one place, dies in another, and then is seen 'haunting' the place where the suffering occurred. All emotions are broadcast, as any empath can attest. And the stronger the emotions broadcast, the longer the 'aura' lingers. Under certain very unusual conditions, including local electromagnetic fields, these emotions can be "etched" into receptive materials which are then "seen" by the psychically receptive, although "read" or "experienced" might be better words.
And the length of time that such 'hauntings' last depending on the surrounding materials which 'record' the psychic/emotional energy? Let's look first at all the materials our technology uses, or has used, to record information. Stone, metal, magnetic media, plastic, paper and ink, etc. Each of them has a different life expectancy. When dealing with psychic energy, the material that absorbs it also has differing abilities. One would expect wood, for instance, to have a lower potential for absorbing and maintaining psychic energy than, say, granite, with its high crystal content. In "haunted" wooden houses, the energy is more likely stored in nails, pipes, foundations, stone fireplaces and/or flooring, etc, which also have crystalline or micro-crystalline structures.
Given that sensitive people can sense and "read" this psychic energy, it follows that it is radiating out of the material that it's stored in. As with any energy, one would expect there to be a limit as to the amount of time it can continue to do so before the energy is exhausted. A perusal of tales of haunted castles and other locations bears this out, more often than not. Eventually, the "ghost" stops appearing. There are exceptions of course, but on average the time seems to be between 100-200 years. There is a possibility that the energy is only expended when the sensitive "taps into" it, which would mean it lasts longer, but how would one be able to test that?
This is a very ethereal (if you'll forgive the pun) area of study, but as with all things concerning the psychic (and often the mundane) world, the more we learn, the more we learn that few things are as simple as we once thought they were. This is one of those areas, I believe. On the other hand, some things are much simpler than we thought they were. *smile*